Samples of potassium hexafluorosilicate (K2SiF6) and polytetrafluoroethylene (C2F4)nwere irradiated in the bremsstrahlung beam from a clinical linear electron accelerator at integral values of endpoint energy between 13 and 32 MeV. Sample positron activities produced by the reactions39K(γ,n)38K,19F(γ,n)18F, and12C(γ,n)11C, were determined by counting the gamma radiation. Saturation activities per target nucleus were calculated from these data. For the K2SiF6samples, the photonuclear activation ratio (PAR) of the 7.6 min39K saturation activity per nucleus divided by the18F result increased by more than 20% per MeV between 15 and 21 MeV. For the (C2F4)nsamples the ratio (PAR) of the11C saturation activity per nucleus to that for18F increased by more than 24% per MeV between 22 and 26 MeV. Quality changes caused by the lead flattening filter were easily detectable using the PAR technique. Because of the high sensitivity to bremsstrahlung spectrum changes these PAR values can serve as sensitive indices of quality for high‐energy beams.